Obviously dildo size is a matter of personal preference above all else. If you have no idea what size you want, start small, but not too small.
Don’t worry about getting a toy that is too long, you don’t need to use the entire length of the dildo. But if you get a dildo that’s too wide, it’s game over, possibly.
The size of dildo someone wants can change, not just over time, but even during sex play. As you become more aroused, and more relaxed, you may want something larger for penetration. For this reason a lot of people have more than one dildo, in different sizes. There are some inflatable dildos available, although most of them are of suspect quality.
